The present invention relates generally to integrated circuits, and more specifically to integrated circuits having increased soft error rate tolerance.
Integrated circuits commonly include storage elements such as latches that retain state information and hold data. During a portion of a time cycle, or clock period, these storage elements hold data to be used during subsequent time cycles. When storage elements reliably retain data, computations can be error free. In contrast, when storage elements do not reliably retain data, computation errors can result.
Cosmic rays and charged particles can cause integrated circuits to be unreliable. When particles bombard portions of integrated circuits, localized areas of charge can build up on an integrated circuit die and cause stored information to be upset. For example, latches having transistors with diffusion regions can be susceptible to bombardment of charged particles. As particles bombard an integrated circuit die about a diffusion region held at a low voltage, the voltage can increase. Likewise, as particles bombard an integrated circuit about a diffusion region held at a high voltage, the voltage can decrease. When the bombardment is significant, the change in voltage in the diffusion region can cause the latch to change state, thereby causing a xe2x80x9csoft errorxe2x80x9d to occur.
The addition of capacitance to a path-exclusive feedback node in a latch circuit is one known method for mitigating the above-described effects. Capacitance provides xe2x80x9ccapacityxe2x80x9d to store a given amount of charge with less voltage change. One drawback of additional capacitance is reduced circuit speed. When the latch circuit changes state, the output voltage value changes, and the additional capacitance is charged as the voltage value changes. Although additional capacitance can reduce the latch circuit""s susceptibility to soft errors, the speed of the latch circuit is reduced in part because the additional capacitance is charged as the voltage value changes.
FIG. 1 shows a prior art latch. Latch 100 includes forward inverter 118 and feedback inverter 110 cross-coupled together. Forward inverter 118 drives feedback node 114 which is input to feedback inverter 110. Feedback inverter 110 in turn drives storage node 112 which is input to forward inverter 118. Latch 100 passes data from data input node 102 to data output node 122 when pass gate 104 is closed. Pass gate 104 is closed when the clock signal on node 108 is high, and the inverse clock signal on node 106 is low. Latch 100 holds data when the clock signal on node 108 is low, and the inverse clock signal on node 106 is high.
When latch 100 is holding data, storage node 112 is at a stable logical state of either logical xe2x80x9c1xe2x80x9d or logical xe2x80x9c0,xe2x80x9d and buffer 120 drives data output node 122. Forward inverter 118 receives the stored data value on storage node 112, and drives feedback node 114 to the opposite logical state than that of storage node 112. Feedback inverter 110 receives the opposite logical state on feedback node 114, and drives storage node 112 with the original stored data value.
Capacitor 116 is coupled to feedback node 114. When charge accumulates on feedback node 114 as a result of cosmic rays or other noise sources, capacitor 116 reduces the voltage variations for a given amount of charge, and reduces the likelihood of a soft error. Along with reducing the likelihood of a soft error, capacitor 116 acts as a low-pass filter, and reduces the speed with which feedback node 114 changes voltage. The addition of buffer (or inverter) 120 allows the data output node 122 to change voltage quickly without regard to the presence of capacitor 116, but also consumes additional area and power.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for improved integrated circuit elements with reduced susceptibility to soft errors.